Method of detecting voltage transients

ABSTRACT

A method and apparatus is provided for resetting an electronic device in response to a voltage transient. The apparatus implements the method steps of detecting the transient at an input to, and receiving an output from, a voltage sensing circuit in response to the transient. The method also includes the step of providing an electronic device reset signal at the output of the voltage sensing device by clamping the input of the voltage sensing circuit to the output of the voltage sensing circuit through an interconnected positive feedback clamping capacitor. Timing capacitors are used to extend the duration of the reset pulse.

FIELD OF THE INVENTION

The invention relates to the detection of voltage transients and morespecifically to a method of triggering reset devices within electricalcircuits requiring such reset devices.

BACKGROUND OF THE INVENTION

Reset controllers and reset control circuits are known. Such controllersmay be found within any microprocessor and microprocessor based circuitsubject to disruption caused by power supply fluctuations. Areas ofapplication of reset controllers may include consumer electronicsequipment such as home computers or any other application whereerroneous results may be associated with supply voltage fluctuations.

Reset controllers may be used within microprocessor circuits to generatea reset signal upon loss of power or upon a detected fault condition.Detected fault conditions may include out-of-spec supply voltages ortransient on a supply voltage power bus that may affect microprocessoroperation.

Prior art attempts to build reset controllers have included thestand-alone use of timing circuits such as the 555 timer or the MaximIntegrated Products, Inc. line of supervisory circuits (e.g. theMAX690). Reset controllers constructed exclusively of 555 timers workwell on start-up by providing a time delay, deferring the start of dataprocessing, until such time as steady-state conditions have beenreached.

Problems arise, on the other hand, with reset controllers constructed of555 timers under conditions of out-of-spec. supply voltages and highspeed transients. A 555 timer may not function (time) properly with asupply voltage of less than 4.5 volts. A 555 timer may also fail todetect high speed transients (glitches greater than 1 MHz in frequency).

Reset controllers using a MAX690 (provided by Maxim Integrated Products)supervisory circuit performs better than a 555 timer under conditions ofout-of-spec supply voltages. Included within the MAX690 is an internalreference voltage supply that may be used in conjunction with externalresistors to detect out-of-spec. supply voltages.

A reset controller using the MAX690 supervisory circuit may achieve adesired reset time delay using an external capacitor. A reset controllermay also achieve the desired time delay through use of additional 555timers.

While the MAX690 performs considerably better than a 555 timer indetecting low voltage conditions the MAX690 is of limited value indetecting high speed transients. The response time of the MAX690, infact, is listed as being 100 μs. Because of the importance of supplyvoltage control for microprocessors and microprocessor circuits a needexists for a reset controller that can detect low voltage conditions andglitches of less than 2.0 μs.

SUMMARY OF THE INVENTION

A method is provided for resetting an electronic device in response to avoltage transient. The method includes the step of detecting thetransient at an input to, and receiving an output from, a voltagesensing circuit in response to the transient. The method also includesthe step of providing an electronic device reset signal at the output ofthe voltage sensing device by clamping the input of the voltage sensingcircuit to the output of the voltage sensing circuit through aninterconnected feedback capacitor.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 depicts a circuit diagram of a reset controller, under theinvention, including a reset circuit and a reset timer.

FIG. 2 depicts operation of the reset controller upon application of atransient.

FIG. 3 depicts a circuit diagram of a reset controller using a variablevoltage reference.

BRIEF DESCRIPTION OF A PREFERRED EMBODIMENT

The solution to the problem of resetting electronic devices during highspeed glitches and low voltage conditions lies, conceptually, in the useof a voltage clamping circuit coupled to a reset timer. The voltageclamping circuit is comprised of a voltage clamping capacitorinterconnected between an input and an output of a means for detectingout-of-spec supply voltage conditions (first voltage sensing device).The output of the first voltage sensing device is then applied as aninput to the reset timer for temporally extending a RESET pulse providedby the voltage sensing circuit.

Shown in FIG. 1 is a reset controller (10), generally, in accordancewith the invention. Included within the reset controller (10) is avoltage clamping circuit and a reset timer. The voltage clamping circuitis comprised of a first voltage sensing device (11), charging resistors(R139, and R232), discharge diode (CR14), and voltage clamping capacitor(C66).

The reset timer is comprised of a discharge diode (CR15), and a firstand second timing RC circuits. The first RC timing circuit includesresistors R233 and R234, capacitor C65, and a second voltage sensingdevice (12). The second timing RC circuit includes resistors R235 andR236, capacitor C63, and an output voltage sensing device (13).

The voltage clamping circuit is beneficially used to trigger the resettimer upon the incidence of voltage transients as short as 1.6 μs. Thereset timer then generates a desired reset delay (e.g. 100 msec.).

The first voltage sensing device (11) may be a MC33064 available fromMotorola Inc. or equivalent. The second voltage sensing device (12) andoutput voltage sensing device (13) may be a MC33161 also available fromMotorola Inc., or equivalent. Discharge diode (CR14) and charging diode(CR15) may be Schottky hot carrier diodes available from Motorola, Inc.

The reset controller (10) may have selected circuit values as follows:

    ______________________________________                                        Device                                                                        Tolerance        Value                                                        ______________________________________                                        R139             162Ω                                                                              1%                                                 R232             560Ω                                                                             10%                                                 R233             392Ω                                                                              1%                                                 R234              51Ω                                                                             10%                                                 R235              51Ω                                                                             10%                                                 R236             221Ω                                                                              1%                                                 C63               3.3 μF                                                                             10%                                                 C65               0.33 μF                                                                            10%                                                 C66              0.047 μF                                                                            10%                                                 ______________________________________                                    

From a deactivated state it can be assumed that the outputs of thevoltage sensing devices (11, 12, and 13) begin in a low state and thatcapacitors (C63, C65, and C66) are discharged. As the power supplyvoltage begins to rise the input of the second voltage sensing device(12) is held in a low state by the output of the first voltage sensingdevice (11).

The output (RESET) of the reset controller (10) is held in a low stateby the output voltage sensing device (13). The output voltage sensingdevice (13) is held in a low state by the output of the second voltagesensing device (12).

As the supply voltage at the input to the first voltage sensing device(11) rises above a first threshold value (minimum device activationvalue of 4.61 volts) the output of the first voltage sensing device (11)changes from a low state to a high state.

As the power supply voltage makes the transition from the deactivatedstate, of 0 volts, to 4.61 volts (and before the first voltage sensingdevice changes state), it can be assumed (depending on the rate of riseof the supply voltage) that the clamping capacitor (C66) charges fromthe deactivated state of 0 volts to +4.61 volts. At the instant that theoutput of the first voltage sensing device (11) changed from a low stateto a high impedance state the clamping capacitor (C66) holds the opencollector output (1) of the voltage sensing device (11) in a low statedue to the 4.61 volt charge. (In the event a negative glitch occurs,discharge diode (CR14) provides a rapid discharge path of the clampingcapacitor, (C66) allowing the voltage sensing device (11) to sense theglitch.) When the voltage sensing device (11) asserts the open collector(1) output (high impedance state), clamping capacitor (C66) dischargesthrough R139 and R232.

Upon activation of the first voltage sensing device (11) a voltage value(clamping capacitor voltage minus discharge diode voltage) (4.50-3.9 v)is presented to the cathode of charging diode (CR15). The voltage valueat the cathode of charging diode (CR15) allows capacitor C65 to begincharging through resistors R233 and R234. Charging of capacitor C65continues until the voltage of capacitor C65 (as presented to the inputof the second voltage sensing device (12)) exceeds a second thresholdvalue (1.27 v) (typical MC33161 threshold voltage) of the second voltagesensing device (12). The time required for the voltage of capacitor C65to rise to the second threshold value can be determined from theequation as follows:

    t.sub.1 =RC*1n(V/(V-V.sub.thr))

where R is a resistance value of the charging circuit, C is acapacitance value, and V_(thr) is a voltage threshold value to which thecapacitor charges. Substituting circuit values, as shown above, providesa minimum charge time for the first RC network of 3.77 msec.

As the second threshold is exceeded a transistor output (6) of thesecond voltage sensing device (12) turns "off" by changing from a stateof low resistance to a state of high resistance. As the first output (6)of the second voltage sensing device (12) moves from low to a state ofhigh resistance, capacitor C63 begins to charge. Capacitor 63 begins tocharge through resister R236 until the voltage across the capacitor(C63) exceeds the second threshold (1.27 v). The time required for thevoltage across capacitor C63 to exceed the third threshold may becalculated using the above equation. Substituting values as shown inFIG. 1 provides a minimum charge time (t₂) for the second RC network of142 msec.

The nominal total time delay upon activation of the reset controller(10) can be calculated by adding the values of t₁ and t₂. Nominal timedelay, under the chosen values, equals 145.77 ms.

While nominal total time delay upon activation provides an indication ofreset time provided by the reset controller (10) the time delay value inuse must be analyzed under worst-case conditions (e.g. a faultcondition). Worst-case analysis, for instance, would require that thetime delay for the second RC circuit be calculated from actual dischargevoltages of C63 and C65.

The actual discharge voltage in the case of C63 is measured by themaximum output voltage of the second voltage sensing device (12) in the"low" state. The typical "low" state voltage of the MC33161 is quoted(device specification) to be 0.2 v.

The actual discharge voltage for C65 is provided by the voltage dropacross CR15 and the first voltage sensing device (11). The maximumdischarge voltage considering typical values is 0.7 v.

Another factor to be considered in worst case analysis is that a RESETpulse from the reset controller (10) for the benefit of external devicessuch as microprocessors and controllers (not shown) should bebeneficially in the 100 ms range under worst-case conditions (e.g. veryfast transients). The 100 ms range under worst-case analysis should bemaintained even including considerations of resistor/capacitortolerances.

Using the values chosen for the second RC circuit (R236=221 Ω, -1% andC63=3.31 μF, -10%) and maximum supply voltage (V_(s) =5.2 v) provides acharge time (from 0 v to 5 v) of 117.46 seconds. Since only 100 ms isneeded a maximum initial voltage for C63 may be calculated through theuse of the equation:

    V.sub.C63 =V.sub.s e.sup.-t/RC

Calculation of the value of V_(C63) provides a maximum initial voltageof 0.2054 mV. Since the calculated value (0.2054 mV) exceeds the maximumoutput voltage (0.200 mV) of the second voltage sensing device (12),operation of the second RC circuit within the area of the 100 ms targetis assured.

A further factor requiring investigation under worst-case analysis isthe discharge time of the second RC circuit. The discharge time of an RCcircuit may be calculated through use of the equation:

    dt=(Cdv)/i

Maximum discharge time using such an equation is 0.99 ms.

Since the maximum discharge time for the second RC circuit is 0.99 ms,the maximum charge time for the first RC circuit (following detection ofa fault) is also 0.99 ms. A calculation of the maximum charge time forthe first RC circuit, using worst case analysis, produces a nominal timevalue of 1.459 ms (from an initial C65 voltage value of zero). Since themaximum charge time for the first RC circuit exceeds the maximumdischarge time for the second RC circuit then a maximum initial voltagefor C65 may be calculated. Using above equation (V_(C) =V_(s) e^(-t/RC))the maximum initial voltage of C65 is 0.907 mV. Since the calculatedmaximum initial voltage of C65 is above the minimum discharge voltageallowed by the discharge path of C65 through CR15 and the first voltagesensing circuit (11), then a minimum charge time for the first RCcircuit of at least 0.99 ms is assured.

The minimum discharge time of the first RC circuit may be calculated asabove. Using worst case analysis produces a minimum discharge time of12.75 μs.

To insure activation of the first RC circuit the voltage clampingcircuit must provide a negative-going activation pulse of at least 12.75μs in duration. The voltage clamping circuit provides such anegative-going pulse (for a reset period) through a charge accumulationon the voltage clamping capacitor (C66).

The voltage clamping capacitor (C66), as mentioned, during steady-stateconditions has zero voltage maintained across capacitor terminals.Should a negative-going transient (fault) occur on the supply voltage,below at least 4.5 v magnitude, the output of first voltage sensingdevice (11) will go to a low state. The low state on the output of thefirst voltage sensing device (11) places a "zero" on the negativeterminal (-) of the voltage clamping capacitor (C66). Since the voltageacross the voltage clamping capacitor (C66) cannot changeinstantaneously, the input (2) to the first voltage sensing device (11)is also "pulled" (clamped) to a low value. The output of first voltagesensing device (11) remains in the low state (following detection of thefault) until such time as the positive terminal (+) of the voltageclamping capacitor (C66) charges to at least 4.5 v through chargingresistor (R139) unclamping the input of the first voltage sensingdevice.

The time required to charge the voltage clamping resistor can becalculated using worst-case conditions for R139 (162 Ω-1%), C66 (0.047μF-10%) and Vs (5.2 V). The calculated minimum charge time for thevoltage clamping capacitor (C66) is 13.6 μs.

Since the calculated minimum charge time of the voltage clampingcapacitor (C66) (13.6 μs) is greater than the minimum discharge time ofthe first RC circuit (12.75 μs), activation of the first and second RCcircuits is assured upon detection of a fault. Activation of the firstand second RC circuits, upon activation of the voltage sensing circuit,assures a reset pulse of 100 ms upon each detection of a fault conditionon the power supply.

Detection of fault conditions (including a transient of 4.5 v, or below)by the voltage sensing circuit is assured by the discharge diode (CR15).The discharge diode remains de-activated until a forward bias voltage of0.6 v appears across the terminals of the discharge diode. If C65 shouldhave charged to 5.0 v, then a negative-going transient of at least 4.4 vwould be required on the power supply to cause forward conduction of thedischarge diode (CR15). Since the voltage sensing circuit detects faultsof at least 4.5 v, detection of faults is assured without interferencefrom the first RC circuit.

Also, since the clamping capacitor (C66), during steady-stateconditions, has zero volts across its terminals, the input (2) andoutput (1) terminals of the voltage sensing device (11) substantiallyfollows voltage transients occurring on the power supply. Since theinput (2) of the voltage sensing device follows the power supply thenperformance characteristics of the voltage sensing circuit substantiallyequals specifications of the voltage sensing device (11).

Performance specifications for the voltage sensing device (11)(Motorola--MC33064) indicate a transient response time as short as 400ns in response to a transient reaching 4.5 v. Experimentation with thereset controller (10), in fact, substantially equal device (11) reactiontimes. Transients as short as 0.65 μs on the power supply, applied tothe reset controller (10), repeatable provide a 100 ms reset pulse atthe output (RESET) of the reset controller (10).

Shown in FIG. 2 is a graphical representation of circuit (10) responseto a high speed transient. As shown a transient on the power supplyreaching 4.5 volts initiates a negative-going RESET pulse ofsubstantially 100 ms in duration. The reset controller (10) thereforeprovides an effective means of detecting and resetting electricalcircuits in the event of high-speed transients.

In another embodiment of the invention (FIG. 3) a reference voltage (24)is provided as an input to the voltage sensing device (25). Thereference voltage (24) is derived within a voltage divider circuit (R3,R4, and R5) from a separate power supply (15 volt supply). The stabilityof the reference (24) is enhanced by a capacitor (C2) and diode (D2).The diode (D2) isolates the reference from the 15 volt power supply. Thecapacitor (C2) maintains the reference voltage (24) during power-down.

The reference output (24) of the voltage divider (R3, R4, and R5) isprovided as an input (-) to a voltage sensing circuit (25). The voltagesensing circuit (25) compares the reference with the output of transientdetector 23, or the supply voltage (+5 V) through optional diode D1.Since the reference voltage (24) may be adjusted by substitution ofvalues within the divider circuit (R3, R4, and R5), the reset controller(20) may be adjusted to detect transients as small as 0.25 volt.

If the transient detector (23) is used, the voltage sensing circuit (20)is activated each time the transient detector (23) senses a transient.Upon activation the voltage sensing circuit (25) is latched in a resetstate by a feedback capacitor (C1). The reset time is determined by anRC timing network (R1 and C1).

Where optional diode (D1) is used (in place of the transient detector(23)), activation of the reset controller (20) may be limited totransients known to affect the circuit protected. With the use of theoptional diode (D1), activation of the voltage sensing circuit (25) isdetermined by a direct comparison of the reference voltage (24) with the+5 V supply voltage. The reference voltage (24) may be adjusted to anappropriate value through substitution of values within the divider (R3,R4, and R5).

We claim:
 1. A method for resetting an electronic device in response to a voltage transient, such method comprising the steps of: detecting, with a voltage sensing device, a supply voltage transient; initiating a reset pulse in response to the supply voltage transient by the voltage sensing device; clamping an input of the voltage sensing device to an output through a clamping capacitor; charging the clamping capacitor to a voltage sensing device threshold value; unclamping the input of the voltage sensing device when an input voltage reaches a threshold voltage; and, terminating the reset pulse by the voltage sensing device when the input to the voltage sensing device is unclamped.
 2. An apparatus for resetting an electronic device in response to a voltage transient, such apparatus comprising: detecting means for detecting a supply voltage transient; means for providing an output, defining the advent of a reset signal, from the detecting means in response to the supply voltage transient; means for clamping an input of the detecting means to the output through a clamping capacitor; means for charging the clamping capacitor to a detecting means threshold value through a charging capacitor; means for unclamping the input of the detecting means upon input voltage reaching a threshold voltage; and, means for providing an output at the output of the detecting means, defining an end of the reset signal, in response to the threshold voltage value at the input to the detecting means.
 3. In a data processing system a reset circuit for detecting voltage transients of relatively short duration, such reset circuit comprising: a voltage sensing device having an input and an output; a diode between the input of the voltage sensing device and a voltage sensing device power supply; a capacitor between the output of the voltage sensing device and voltage sensing device ground; and, a positive feedback clamping capacitor between the input and output of the voltage sensing device.
 4. The circuit as in claim 3 further comprising a resistor between the input of the voltage sensing device and the voltage sensing device power supply.
 5. The circuit as in claim 3 further comprising a resistor between the output of the voltage sensing device and the voltage sensing device power supply. 